1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser beam printer or the like.
2. Related Background Art
The image bearing member, employed in a four-colored full-color image forming apparatus of the electrophotographic system (for example a copying machine or a laser beam printer), is an electrophotographic photosensitive member composed of a metal drum surfacially provided with a photosensitive layer such as an organic photosensitive coating (OPC), and is rotated in a predetermined direction in response to a print start signal. The image bearing member, thus rotated, is surfacially charged uniformly to a predetermined potential, by the application of a charging bias voltage to a charger. Then the surface of the image bearing member is exposed to (irradiated by) the light of a specified wavelength, based on a signal from a controller. The electrostatic bharge in the irradiated portion is dissipated, whereby an electrostatic latent image is formed on the surface of the image bearing member.
Subsequently, a yellow developing unit, containing yellow toner and so positioned as to oppose to the image bearing member, is given a certain developing bias voltage, whereby the toner given a predetermined charge is deposited onto the electrostatic latent image present on the image bearing member to develop a visible toner image. Then the toner image on the image bearing member is subjected to a primary transfer onto an intermediate transfer member (for example intermediate transfer belt) positioned adjacent to the image bearing member and driven with a substantially same speed as that of the image bering member, by a primary transfer bias voltage of a polarity opposite to that of the toner present on the image bearing member.
The remaining toner of the primary transfer, which is not transferred to the intermediate transfer member in the above-mentioned primary transfer step but remains on the image bearing member, is removed by a cleaning device. The cleaning device is provided with a cleaning blade (cleaning member) of which end is maintained in counter contact with the rotating direction of the image bearing member (forming an acute angle with the tangential line at the downstream side in the moving direction) and mechanically scrapes off so-called "transfer remainder toner" which remains on the image bearing member after the primary transfer.
The above-explained steps are repeated also for the toners of other colors, namely of magenta, cyan and black, whereby toner images of four colors are superposed on the intermediate transfer member.
Subsequently a secondary transfer bias voltage of a polarity opposite to that of the toner images is applied to a secondary transfer device opposed to the intermediate transfer member and a transfer material such as paper is passed in this state between the intermediate transfer member and the secondary transfer device, whereby the toner images of four colors present on the intermediate transfer member are collectively transferred onto the transfer material. In this operation, so-called "secondary transfer remainder toner" which remains on the intermediate transfer member is removed by an intermediate transfer member cleaning member. For cleaning the intermediate transfer member, there are known a method of contacting a blade or a brush with the intermediate transfer member and mechanically scraping off the remainder toner, and a method of inversely charging the remainder toner on the intermediate transfer member, then re-transferring such remainder toner onto the image bearing member at the contact portion thereof with the intermediate transfer member and removing thus re-transferred toner by the cleaning member for the image bearing member.
The transfer material bearing the toner images of four colors transferred by the secondary transfer is transported to a fixing device, in which the toner images are fixed to the surface of the transfer material under the application of heat and pressure.
The transfer remainder toner (hereinafter collectively meaning the primary transfer remainder toner and the secondary transfer remainder toner) deposited on the surface of the image bearing member is removed by the cleaning blade which is so mounted that the end thereof is in counter contact with respect to the moving direction of the image bearing member. In the rotating state of the image bearing member at a constant speed (hereinafter also called "normal rotation state"), the image bearing member and the cleaning blade are maintained under a mutually tensioned constant state in which no gap is present therebetween, whereby the transfer remainder toner deposited on the surface of the image bearing member can be cleanly removed.
However, when the rotation of the image bearing member is stopped or started, the contact between the image bearing member and the cleaning blade becomes a state that is different from the normal state. More specifically, in the normal rotation state, a frictional force based on the dynamic friction coefficient is generated between the image bearing member and the cleaning blade, but, at the start of movement of the image bearing member, there is added a frictional force based on the static friction coefficient. A shock resulting from such sudden change of the state induces "fine vibration phenomenon" of the cleaning blade. As a result, there is generated a fine gap between the image bearing member and the cleaning blade, whereby the transfer remainder toner, that is accumulated in the vicinity of the cleaning blade, temporarily passes under the cleaning blade. The transfer remainder toner, that has passed under the cleaning blade, is transferred to the intermediate transfer member, and further therefrom to the transfer material, thus eventually inducing deterioration of the quality of the formed image.